Quantum Chemical Modeling of Asymmetric Enzymatic Reactions : Applications to Limonene Epoxide Hydrolase and Arylmalonate Decarboxylase

Sammanfattning: In this thesis, density functional theory has been employed to study the reactionmechanisms of two enzymes with possible applications in asymmetric biocatalysis.To reproduce and rationalize the stereoselectivity of the enzymes, quite large cluster models that account for the chiral environment of the active site have been used.In the first study, the enantioselectivity of the wild-type limonene epoxidehydrolase and two groups of mutants thereof, that show either (R,R)- or (S,S)-selectivity, were investigated. Using the cluster approach, the enantioselectivity for each variant of the enzyme was calculated and the results are in good agreement with the experimental data. It was found that the enantioselectivity of the enzyme variants is controlled by the steric hindrance introduced or relieved bythe different mutations.The second study concerns the reaction mechanism and stereoselectivity of arylmalonate decarboxylase. The calculations support the proposed two-step mechanism, in which decarboxylation and protonation of the substrate occur separately. The stereoselectivity of the enzyme is governed by repulsive steric interactions between the substrate and the residues that deffine a large and a small cavity in the active site. Depending on the size of the substrate, the selectivity was found to be determined already at the binding of the substrate or in the subsequent transition state.The results presented in this thesis demonstrate that the quantum chemical cluster approach for modeling enzymes is indeed a very valuable tool in the study of asymmetric biocatalysis.